1. Field of the Invention
This invention relates to the oxidation of thallium (I) to thallium (III).
2. Description of the Prior Art
Trivalent thallium compounds, i.e., thallic compounds, have been used as oxidizing agents in various reactions. For example, Kruse et al., J. Org. Chem.36, 1154 (1971) describe the epoxidation of certain olefins with thallic acetate and U.S. Pat. No. 3,641,067 relates to the preparation of the epoxides of propylene and isobutylene by means of lower thallic alkanoates.
In all of these reactions the trivalent thallium is reduced to the monovalent state and, if the thallium is to be reused in the reaction, it is necessary to reoxidize or "regenerate" it by converting thallium (I) to thallium (III). Various methods for effecting this conversion have been proposed and are more or less effective. Thus, U.S. Pat. No. 3,399,956 (issued to Hirose et al.) describes the oxidation of Tl(I) to Tl(III) by means of molecular oxygen in an acidic aqueous medium containing chloride or bromide ions and an ion of a redox metal such as copper, mercury, chromium, manganese, iron, cobalt, and nickel. Hirose et al. refer to earlier processes for effecting the conversion of Tl(I) to Tl(III) and point out the problems involved in achieving the desired oxidation and the disadvantages and drawbacks of prior procedures. While the Hirose et al. process is described as an improvement over processes previously proposed, it is limited to the use of aqueous chloride or bromide solutions so that the thallium (III) is always produced as a chloride or bromide and it is generally necessary to use the redox metal in large amounts in relation to the thallium compound being treated.
The disclosure in J. F. Spencer, Z. Anorg. Chem., vol 44, p. 379 (1905) of the oxidation of a thallous nitrate to the thallic state using molecular oxygen in a 1 N. acid solution in the presence of platinum plate which had been coated with platinum black is only of academic interest due the limited conversion obtained (only 1 to 2%). Moreover, such low conversion was obtained only after repeated polarization of the platinum plate. U.S. Pat. No. 3,479,262 relates to the conversion of thallium (I) to thallium (III) by use of an oxidizing agent having a formal oxidation potential level of about 1.3 to less than 1.5 volts, particularly cerium (IV) sulfate, in the presence of a noble metal salt. Japanese Patent Publication 74/13, 104 (published Feb. 5, 1974) relates to the epoxidation of C.sub.3 to C.sub.5 olefins by use of oxygen or air together with thallous salts, aliphatic acid salts of alkali or alkaline earth metals, halides of alkali or alkaline earth metals and optionally halides or aliphatic acid salts of copper or iron, in aqueous media. Japanese Patent Publication 76/44,116 (published Nov. 26, 1976) relates to the oxidation of thallium (I) to thallium (III) using molecular oxygen in aqueous medium in the presence of chloride and/or bromide of a redox metal.
U.S. Pat. No. 3,436,409 relates to the regeneration of thallic oxide from thallous oxide by "known techniques" as by chemical methods of reaction with molecular oxygen, hydrogen peroxide and the like or by electrolytic methods. Supporting electrolytes such as water-soluble inorganic salts and water-soluble organic salts are disclosed for use in connection with the electrolytic methods. Tetramethyl ammonium iodide is stated to be a suitable water-soluble organic salt for use as a supporting electrolyte.